Gaseous fueled automobile engines, that is, those to which fuel is metered to the engine in the gaseous form as opposed to the conventional liquid form, have been investigated for production purposes for several reasons. First, fuels for gaseous fueled engines, for instance natural gas consisting primarily of methane, has a much higher domestic production rate than petroleum. Thus, domestic dependence on foreign oil supplies would be reduced as gaseous fueled engines proliferate. Second, natural gas is a renewable energy source. For example, a landfill can continuously produce methane as new waste is decomposed. Third, the price of natural gas is typically less than petroleum on an equivalent energy basis. Fourth, natural gas generally burns cleaner than liquid hydrocarbon fuels reducing significantly the carbon deposits prevalent with such liquid fuels. As a result, less maintenance is required. Fifth, natural gas has a higher octane rating which can provide better performance and economy. Sixth, natural gas is a more stable fuel, from both a chemical standpoint and since its ignition temperature is higher than petroleum. Natural gas also dissipates quicker and, should it leak from the automobile, it would rise since it is lighter than air thus making it less a hazard for accidental ignition than liquid fuels.
For the aforementioned reasons, gaseous fueled engines are an attractive alternative to liquid petroleum operated engines. Fuel for a gaseous fueled engine is typically stored in a canister at high pressure, for example 300-3000 pounds per square inch (psi), and fed to the engine through a fuel line. A pressure regulator reduces the stored pressure to a pressure satisfactory for metering into the engine, typically in the range of 100 psi. One problem with such an arrangement for gaseous fueled engines is the production of a pressure wave within the fuel rail and the fuel supply line during fuel metering to the engine cylinders. This pressure wave, which has an amplitude of approximately 3 to 5 psi above and below existing fuel rail pressure, is caused by the successive opening and closing of fuel metering devices to the cylinders. The pressure wave potentially causes uneven fuel metering from the fuel injectors to the cylinders since the amount of fuel forced to the cylinders varies with rail pressure. Undesirable results of uneven fuel metering are a rough idle, loss of fuel economy, poorer exhaust emissions, and, in severe cases, deteriorated drivability. This uneven fuel metering results partially from the pressure wave travelling, at the speed of sound through the fuel, to the pressure regulator. The pressure regulator attempts to compensate for the pressure variation and, if the rarefaction wave frequency corresponds to the pressure regulator's natural frequency, extreme pressure fluctuations result.
For any engine load, the fuel metering devices will request a corresponding air-fuel ratio. When the pressure wave passes unsuppressed through the fuel rail, the actual air-fuel ratio delivered to the engine varies from that requested due to gaseous pressure differentials. The primary objective of the fuel metering system is to provide an actual air-fuel ratio that corresponds as closely as possible to that requested, typically by the engine on-board computer. Obviously, any fluctuations in rail pressure which occur faster than the rail pressure sensor and computer can respond will cause uneven fuel metering.